The present invention relates to a fibre-reinforced ceramic body and a method for producing the same, in particular a brake disc of fibre-reinforced ceramic material and a method for producing the same.
Fibre-reinforced ceramic bodies and methods for producing them are known. For example a brake disc is known from DE-A-44 45 226 that is produced from a carbon fibre-reinforced material in two halves as an internally ventilated brake disc with radial ventilation channels.
The two surfaces of the brake disc are formed as friction surfaces. Ribs are formed on the inside of at least one half that rest on the other half with the formation of free ventilation channels arranged therebetween. Both halves are rigidly joined to one another by a suitable joining method, such as for example high-temperature brazing or an adhesion method process. A pot-shaped flange that is either formed integrally with one half or is connected via bolts or other joining elements to the brake disc serves for the purposes of securing to the wheel.
Such brake discs may consist for example of a carbon fibre-reinforced carbon (CFC) and have been used for some years with specially developed friction linings in motor racing. The area of use of CFC materials is however limited on account of the susceptibility of the carbon fibres to oxidation starting at about 500xc2x0 C. Moreover, further composite ceramic materials have become known, from which such brake discs or other ceramic bodies can be produced (see for example DE-A-197 11 831). In this connection long fibre-reinforced or short fibre-reinforced CFC materials are used as preliminary bodies, which are then melt infiltrated with liquid silicon. Reaction-bonded carbon fibre-reinforced SiC ceramics are thereby formed.
In general fibre-reinforced ceramic materials are used as high performance materials in machine construction and plant construction and in aeronautics and space technology. A good tolerance to damage and resistance to thermal shock and oxidation, in addition to a very high specific strength and rigidity, are desired in such applications. The aforementioned properties are restricted when using long fibres since an oxidative attack can penetrate into the interior of the structural part and thereby lead to destruction if the critical crack length is exceeded.
In the case of short fibre-reinforced ceramic bodies with reaction-bound fibres based on Si/C/B/N according to DE-A-197 11 831, a better oxidation stability can, in fact, be achieved even at high temperatures since oxidation of the short fibres that are present in isolated form due to the ceramic matrix damage the structural part only on the surface. However, such ceramic bodies in many cases do not exhibit a sufficient tolerance of damage such as is required for certain high performance applications, for example for brake discs.
The object of the present invention is accordingly to provide an improved fibre-reinforced ceramic body and a method for producing such a body that is characterised by a high tolerance of damage, high strength, and high temperature resistance. In this connection, it should be possible to produce a surface that is as gas-tight and/or liquid-tight as possible to ensure a good corrosion resistance.
This object is met by a fibre-reinforced ceramic body with a novel structure that consists of a core and a boundary layer that is joined to the core and has at least one outer surface that can preferably be subjected to tribological stress, with the core being composed of one or more layers, of which at least one is reinforced with long fibres, and wherein the boundary layer is reinforced with short fibres.
With regard to the above method, this object is achieved by a method in which a core is produced from at least one layer that is reinforced with long fibres and the core is joined to a boundary layer that is reinforced with short fibres and has at least one outer surface that can preferably be subjected to tribological stress.
The object of the invention is fully achieved in this way. According to the invention a high tolerance damage is in fact achieved by means of the core that is reinforced with long fibres, while on the other hand a high temperature resistance can be achieved by means of the boundary layer reinforced with short fibres since the said boundary layer can be made gas-tight and liquid-tight by a large proportion of ceramic matrix and by using short fibres, with the result that a good corrosion resistance can be achieved also at temperatures above 500xc2x0 C.
In a further modification of the invention the core has a plurality of layers reinforced with long fibres (UD layers), whose long fibres are in each case arranged in a preferred direction, with the preferred directions of at least two UD layers being arranged angularly displaced relative to one another.
In this connection, layers with woven long fibres may additionally be provided, which are preferably arranged between adjacent UD layers in order to counteract delamination.
The strength properties of the ceramic structural part can be purposely influenced by means of the combination of a plurality of UD layers arranged at an angle relative to one another and optionally layers of woven long fibres (cross-ply layers), in order thereby to achieve a particularly high strength in the specific stress directions acting on the ceramic structural part and at the same time ensure a good tolerance of damage.
In this connection the short fibres are preferably arranged statistically distributed in the boundary layer.
In order to achieve such a homogeneous distribution of the short fibres, a granulation method may for example be employed.
The term long fibres within the scope of the present application is understood to mean fibres having a length of at least 50 mm, while the term short fibres is understood to mean fibres having a length of less than 50 mm.
Such fibres are used, as a rule, in the form of fibre bundles (so-called rovings), which generally consist of ca. 3,000 to 25,000 individual filaments. 12K rovings, which consist of about 12,000 individual filaments, are widely available commercially.
All fibres that have a sufficiently high strength even at high temperatures, as long as exclusion of air is ensured, are suitable as fibres for reinforcing the ceramic structural part. Such fibres are as a rule those with a covalent bond based on silicon, carbon, boron and/or nitrogen. SiC fibres, C fibres or SiBCN fibres may preferably be used as fibres.
In an advantageous development of the invention the fibres are reaction-bonded with a matrix that may, in particular, consist of silicon carbide. The use of such a matrix material permits an advantageous production and at the same time provides a good gas-tightness in the boundary layer, and, furthermore, problems that may arise due to shrinkage during production can be largely avoided.
Such a ceramic body may be designed and fabricated as a brake disc for an internally ventilated disc brake, and may comprise two halves joined to one another, each half having an outer surface formed as a friction surface and an inner surface, wherein at least one half has on the inner surface ribs that rest on the other half, and wherein on the inner surfaces of both halves ribs are provided that run substantially radially and that engage positively between corresponding ribs of the respective other half.
With such a design of a brake disc consisting of two halves, the securing to the wheel can be considerably simplified due to the positive and material-locking connection of the interengaging ribs, since a relative rotation of the two halves can be effectively prevented by the said positive and material-locking connection. Accordingly, the securing of the brake disc to the wheel can be effected in a particularly simple manner, in which connection if necessary just a simple friction-locking connection may also be used in order to completely prevent the propagation of notch stresses into the brake disc. Overall, the stability of the brake disc consisting in such a way of two halves is furthermore substantially improved compared to known brake discs.
In a preferred modification of the invention, ventilation channels are formed between the ribs of the one half and the ribs of the other half.
Such ventilation channels, which particularly when the brake disc is produced from a composite ceramic material are advantageous on account of the relatively poor thermal conductivity of the ceramic material, can be incorporated into the brake disc according to the invention without additional expenditure.
The ventilation channels may be designed straight and to run in a radial direction. In addition it is of course also conceivable for the ventilation channels to follow a curved, for example spiral, path.
In a preferred modification of the invention both halves of the brake disc are designed to be identical as regards shape and size.
The production costs involved in the manufacture of the brake disc can be significantly reduced by means of such a geometry. Thus, a single press tool can be used to produce both halves, thereby halving the tooling costs.
In a preferred modification of the invention each rib has a first supporting surface running in a radial plane, which changes via an inclined surface into a second supporting surface likewise running in a radial plane, the said second supporting surface being at a greater distance from the friction surface than the first supporting surface.
Such a design ensures in the brake disc an as good and uniform an application as possible of the braking pressures acting externally on the two friction surfaces, since due to the plane supporting surfaces large cross sections are available for the transmission of the force. The inclined surfaces provided therebetween, which may run either perpendicularly to the radial plane, inclined thereto or may also be formed as curved surfaces, serve for the positive engagement of one half on the other half.
According to an advantageous modification of the invention grooves that run substantially radially are formed on the inside of each half, between which grooves the ribs project, and wherein four ribs are in each case arranged in succession to one another in the circumferential direction in such a way that the first supporting surfaces of the first and of the adjacent second rib point towards one another, while the first supporting surfaces of the second and of the following third rib point away from one another, and the first supporting surfaces of the third and of the following fourth rib again point towards one another.
By means of such an arrangement it is possible for both halves to be identical in both shape and size. However, in principle other geometries that permit an identical design and arrangement of the two halves is also conceivable.
In a preferred modification of the invention, the brake disc has a central securing opening for securing by means of a flange to a wheel, with the friction surfaces being designed so as to be continuously planar to the friction-locking connection with the flange or have grooves starting from the securing opening for the positive connection with the flange.
This arrangement has the advantage that with just a friction-locking connection between the flange and the brake disc, no types of notch stresses are transmitted from outside to the brake disc. However, it is in principle also possible to use grooves starting from the securing opening for the positive connection with the flange, if a continuously positive connection is desired between both halves of the brake disc and the mounting on the wheel.
A brake disc according to the invention can be used in a manner known per se in a brake disc that has a brake saddle that externally encloses a specific region of the said brake disc on both sides and that has brake linings that can be pressed against the friction surfaces.
In a preferred modification of the invention the ribs of the brake disc are spaced apart from one another in such a way that the brake linings cover at least two ribs of the brake disc.
Such a design ensures that the pressing force acting from both sides on the brake disc is applied as uniformly as possible to the brake disc and that buckling or bending moments are largely avoided.
In an advantageous modification of this embodiment the flange has two partial flanges that in each case partially cover the friction surfaces of the brake disc with a plane annular surface on the inside, and wherein clamping means are provided for tensioning the partial flanges against one another, in order to ensure a friction-locking connection between the brake disc and the inner flange.
In this way a particularly smooth torque transmission between the brake disc and the wheel is ensured without notch stresses possibly being transmitted to the brake disc by bolts or the like, this having a particularly advantageous effect on safeguarding against the danger of fracture in production from composite ceramic materials.
According to an alternative embodiment grooves starting from the connection opening are provided on at least one friction surface, corresponding flange webs positively engaging in the said grooves.
Such a design is advantageous if a continuously positive connection between the two halves of the brake disc and the securing to the wheel is desired.
In a modification of the production process according to the invention the composite ceramic material is produced by the following steps:
production of a prepreg formed from at least one layer of long fibres which is mouldable while under the action of heat while subject to the addition of organic precursors and preferably while subject to the addition of fillers;
production of a mixture formed from short fibres and organic precursors which is mouldable while subject to the action of heat preferably under the addition of fillers;
introduction of at least one prepreg together with a layer consisting of the mixture into a mould and compression under the action of heat to produce a green product;
pyrolysis of the green product to produce a porous moulded body;
melt infiltration of the porous moulded body with a melt, preferably with a silicon melt, to produce a moulded body with reaction-bonded fibres.
In this way the production of the ceramic body can be considerably simplified since the prepreg can be produced by means of a standardised production process and then only has to be moulded to the desired shape under the action of heat together with the mixture containing short fibres. The moulded body formed in this way can be fabricated roughly having its final contour and shape, so that after the subsequent pyrolysis and melt infiltration only a minor post-treatment is necessary.
In this connection preferably a plurality of prepregs, which may contain long and/or short fibres, are assembled to form a laminate that is then shaped in the mould under the action of heat.
In this connection the mixture of short fibres and organic precursors, to which fillers are preferably added, may either be incorporated into one or several prepregs and then laminated with the prepregs reinforced with long fibres, or may be produced separately, for example by synthesis granulation, in order to obtain a boundary layer as homogeneous as possible reinforced with short fibres.
The mixture from which the boundary layer is produced may also already be incorporated into the relevant outer layer of the long fibre prepreg. In order to achieve greater layer thicknesses, several prepregs that are reinforced exclusively with short fibres are preferably used. Thus, a plurality of prepreg layers may be laminated (placed over one another) and optionally various UD layers and woven layers may be combined with one another by laying various prepregs on top of one another.
In principle various processes may be used to produce the prepreg.
A particularly favourable process for producing a prepreg according to the invention has proved to be the SMC (xe2x80x9csheet moulding compoundingxe2x80x9d) method which in principle is already known for producing glass fibre-reinforced composite materials but not however for producing fibre-reinforced ceramic materials.
For this purpose a carrier film is unwound from a storage roll, an organic precursor and fibres are then added from above to the carrier film, finally a further organic precursor and a further carrier film are added from above, and the material enclosed between the two carrier films is compacted and wound onto a storage roll. The precursor is next aged, which may take place at room temperature over a period of a few hours to several days, in order thereby to achieve a thickening effect and/or a partial crosslinking. After sufficient ageing of the precursor, the two carrier films may be removed from the two sides of the prepreg and the prepreg may then be cut into sections for the production of the green product. By laminating several prepregs a multilayer structure is obtained that is compressed in a suitable mould.
According to the SMC process the fibres are unwound in the form of long fibres together with the organic precursor. If short fibres are to be produced, the fibres may also be cut to the desired length by means of a cutting device (chopper) during the prepreg production, following which they are incorporated into the organic prepreg layer.
In this connection it is possible to combine a long fibre-reinforced layer with a short fibre-reinforced layer in a single production process.
Alternatively, the short fibre-reinforced layer for the boundary region may of course also be produced in a different way (for example by granulation) and then combined later with the long fibre-reinforced prepreg.
A further variant of the production of a green product is the so-called RTM process, which is likewise known for the production of glass fibre-reinforced plastics.
According to this process variant a fibre framework of long fibres is placed in a mould, an organic precursor is next injected into the mould under pressure, and the whole is moulded. According to the invention at least one layer comprising a mixture of organic precursor and short fibres and optionally fillers is now applied to the body formed in this way, the said body then being post-moulded in a mould under the action of heat before the green product thereby produced is pyrolysed and finally melt infiltrated. As an alternative it is also conceivable to inject short fibres together with the organic precursor into the mould.
In this way the shaping and the fibre-matrix integration is thus carried out in one stage.
A pitch or a resin, in particular a phenol resin or a furan resin, is preferably used as organic precursor for the aforementioned processes.
A further variant in the production of a long fibre-reinforced prepreg is the so-called slip infiltration.
To this end long fibre rovings are unwound and infiltrated by means of an immersion process with a slip consisting of an organic or organometallic precursor, preferably a polysilane, a polysiloxane or a polycarbosilane, as well as with solvents and fillers, and then wound onto a plaiting core.
The prepreg produced in this way may then be combined with short-fibre reinforced compositions in a pressing tool and next processed further in the already described manner, in order to produce a suitable structure having short fibre-reinforced and long fibre-reinforced layers.
The previously described production process may be used for example to produce a one-piece brake disc or an internally ventilated brake disc that is composed of two halves.
If the brake disc is produced from two halves, then preferably at least one prepreg that has at least one long-fibre-reinforced layer is introduced together with a short fibre-reinforced mixture into a pressing tool and is compressed, under the action of heat, to produce a first half of the brake disc, while at least one further prepreg is introduced into preferably the same pressing tool and is compressed under the action of heat in order to produce the second half of the brake disc. The two halves are then pyrolysed under the exclusion of air to produce a porous moulded body that is next melt infiltrated with a reactive melt, preferably with a silicon melt.
By joining the two halves before the pyrolysis and melt infiltration with a silicon melt of the porous moulded body produced by pyrolysis, a particularly intimate joining of the two halves to one another is achieved since to some extent fibres of one half hook onto the surface of the other half and during the subsequent melt infiltration even both halves are to some extent joined to one another in a material-locking manner by means of the continuous metallic inclusions that have penetrated into the porous skeleton of both halves. Moreover, a particularly intimate joining of the two halves is achieved by the mechanism of the reaction bonding between silicon and carbon. During the melt infiltration SiC also present in the green product at least partially redissolves and separates out as secondary SiC during the subsequent cooling, which in addition improves the bonding of the two halves.
It is understood that the expression xe2x80x9csilicon meltxe2x80x9d is also understood to mean such a melt that contains, in addition to silicon, alloyed constituents, in particular iron and chromium, which is particularly preferred on account of the considerably reduced internal stresses as a result of volume changes, as is discussed in detail in DE-A-197 11 831.
It is understood that both the remaining aforementioned features and the following features still to be discussed in detail may be used not only in the combination specified in each case, but also in other combinations or even alone, without going beyond the scope of the present invention.